Process and device for producing plastic moldings from a flowable reaction mixture

Abstract

Polyurethane molded articles are produced from a flowable polyol-polyisocyanate reaction mixture on a shot-wise basis. The flowable reaction mixture is formed by combining the components of that mixture in the mixing chamber of a mixhead. The contents of the mixhead are introduced (preferably via an outlet pipe) into a mold in a manner such that the mixing chamber is substantially emptied in each case. Any ambient gas in the mixing chamber is then removed before the respective next shot is initiated.

What is claimed is:  
     
       1. A process for the production of a molded polyurethane article from a flowable polyol-polyisocyanate reaction mixture present in a mixing chamber comprising 
       a) introducing the flowable reaction mixture into a mold in a manner such that the mixing chamber is substantially emptied,  
       b) removing any ambient gas present in the mixing chamber after step a) has been completed but before step a) is repeated, and  
       c) replacing the ambient gas removed in step b) with a gas that is soluble in the flowable reaction mixture.  
     
     
       2. The process of  claim 1  in which a closure element located at the outlet end of the mixing chamber is closed before step a) and is opened only at the moment at which the mixing chamber is filled with the flowable reaction mixture. 
     
     
       3. The process of  claim 1  in which carbon dioxide is used as the soluble gas. 
     
     
       4. The process of  claim 1  in which the mixing chamber is flushed with a gas that is soluble in the flowable reaction mixture before step a). 
     
     
       5. The process of  claim 1  in which the vacuum is generated by connecting the mixing chamber to a vacuum container and/or a vacuum pump. 
     
     
       6. The process of  claim 2  in which the mixing chamber is a counterflow high pressure mixing chamber with an ejector rod arranged in the mixing chamber, the ejector rod is moved up as far as the closure element prior to step a) and a vacuum is generated by retracting the ejector rod with the closure element closed. 
     
     
       7. The process of  claim 1  in which the mixing chamber tapers conically in the direction of flow of the reaction mixture and has a correspondingly conical rotor movable in an axial direction. 
     
     
       8. The process of  claim 7  in which the conical rotor is moved axially before step a) in a manner such that the mixing chamber volume is approximately zero, a closure element located at the outlet of the mixing chamber is closed, and the rotor is returned to the working position to create the vacuum. 
     
     
       9. A device useful the production of a molded polyurethane article by a shot process comprising: 
       a) a mixhead having a mixing chamber with an inlet and outlet opening present therein,  
       b) means for introducing reaction mixture components into the mixhead,  
       c) means for conveying any content of the mixing chamber into a mold,  
       d) a mold for receiving the contents of the mixing chamber,  
       e) a gas outlet for the removal of ambient gas present in the mixhead after substantially all reaction mixture has been removed from the mixing chamber; and  
       f) means for introducing gas into the mixhead after removal of the ambient gas.  
     
     
       10. The device of  claim 9  in which the means for conveying the contents of the mixing chamber to the mold is an outlet pipe. 
     
     
       11. The device of  claim 9  in which the means for introducing reaction mixture components into the mixing chamber is an injection nozzle. 
     
     
       12. The device of  claim 9  in which a closure element is present at the outlet of the mixing chamber.